This invention relates to the selective addressing of groups of broadcast satellite receivers within a physical geographic portion of the total satellite footprint.
In satellite broadcasting there is an increasing desire for the broadcaster to be able to transmit different items to receivers in different geographical locations, that is, to provide location-aware services. With terrestrial broadcasting the problem is simply solved by transmitting the different items on different transmitters that have a limited geographical range. However, geostationary satellite transmitters have a footprint on the earth's surface that can extend to many hundreds of kilometers or even miles. Such a footprint is too large from the point of view of the broadcaster who may wish to transmit different items to receivers in different localities within that footprint.
Thus a need has been recognized for a receiver to be able to adopt a behaviour based upon its own location. Currently most receivers have a separate “set-top box” or decoder which decodes the signal, and in such cases the recognized need is for the set-top box to be able to receive services exclusively aimed at its own location. Currently such receivers and set-top boxes do not, in general, “know” where they are geographically within the satellite footprint.
Subscription television (Pay TV), or more accurately conditional-access television, which uses subscriber cards in the receiver that are individually prepared for each subscriber, overcomes this problem automatically. This is because each receiver has its individually-prepared subscriber card carrying decoding keys or the like, and these keys can be selected in dependence upon the receiver location so as to enable the receiver to receive the transmissions intended for its locality and only those transmissions. Not all location-aware services are however intended for transmission on subscription channels or to receivers equipped with such subscriber cards. Furthermore, a subscription television service of this type requires all the complexity of a subscriber management system, and there are other disadvantages to using conditional-access systems.
Thus we have appreciated that there is a need for a system which can address all the receivers of a geographical group substantially without addressing receivers not in that geographical group.
Clearly one possibility would be to include within each broadcast receiver a GPS receiver which makes use of the Global Positioning System satellite network. However, it is not feasible to require all broadcast receivers, or rather their “set-top boxes” or decoders, to be fitted with a GPS receiver.
Whatever system is used should not be one that can be easily “hacked”. In particular it should not be one that can be circumvented simply by sending a key over the internet.
International Patent Application WO02/097470 describes a method of addressing a location based on latitude and longitude by transforming it into a universal location string, and uses the global positioning satellite system (GPS). U.S. Pat. No. 6,560,340 provides conditional access to encrypted cable TV services, based on entitlement information received with the service. International Patent Application WO03/043323 describes the use of digital TV broadcast signals to provide GPS-aiding information. The Abstract of Japanese Laid-Open Patent Publication 11-187013 (equivalent to Chinese Laid-Open Patent Application 1224962) describes a cryptographic key distribution system based upon a binary tree. With a system of this type groups of broadcast receivers can share a common root and be addressed as a group by encrypting the message with the common root. European Patent Application 420329 describes a GPS receiver utilizing the P-code signals of the NAVSTAR GPS system. International Patent Application WO00/67450 describes methods for determining, collecting and using the geographic locations of Internet users, the determination being made by detecting the route taken in delivering packets to the user.